Abstract: Ewing sarcoma is the second most common pediatric bone cancer. These tumors are particularly malignant, as 25% of patients will be metastatic at the time of diagnosis. The five- year survival rate for patients with primary Ewing sarcoma is 64% and less than 30% for patients with disseminated disease. Nearly all of Ewing sarcomas are caused by a translocation between either EWS or FUS and FLI1, creating a powerful tumor-causing fusion protein, EWS-FLI1 or FUS-FLI1. FUS, EWS, and TAF15 are transcription factors that together form the FET family of proteins. We will explore interactions between FET proteins and fusion proteins in Ewing sarcoma that regulate transcription and thereby promote tumor formation. We propose that large transcription granules formed by FET proteins serve to localize transcription machineries to active sites of gene expression. We hypothesize that fusion proteins formed from FUS or EWS drive transcription granules to constitutively form at poly-GGAA sites in the genome. Launching our investigation into the basic molecular mechanisms by which FET fusion proteins contribute and drive pediatric sarcoma initiation, progression, and metastasis, we propose two aims. The goal of these aims is to (1) test a model that fusion proteins work hand-in- glove with the wildtype variants to reprogram cells toward disease and (2) establish the basic biochemical frameworks for FET-fusion structure/function relationships in our in vitro transcription assays. The result of these studies will well position us to identify and investigate key interactors that modify FET and FET fusion protein activity in the disease-cellular environment.